Precise determination of polarization fields in c-plane GaN/AlxGa1-xN/GaN heterostructures with capacitance–voltage-measurements

Norman Susilo,Ingrid Koslow,Monir Rychetsky,Michael Kneissl,Bernd Witzigmann,Martin Guttmann,Xiaohang Li,Tim Wernicke,Georgios G Roumeliotis,Michael Lehmann,Johannes Enslin,Tore Niermann,Hsin-Hung Yao,Marcel Schilling,Michael Narodovitch

doi:10.7567/1347-4065/ab09dd

Abstract

Due to changes in the spontaneous and piezoelectric polarization, AlGaN/GaN heterostructures exhibit strong polarization fields at heterointerfaces. For quantum wells, the polarization fields lead to a strong band bending and a redshift of the emission wavelength, known as quantum-confined Stark effect. In this paper the polarization fields of thin AlGaN layers in a GaN matrix were determined by evaluating the changes in the depletion region width in comparison to a reference sample without heterostructure using capacitance–voltage-measurements. The polarization fields for Al0.09Ga0.91N (0.6 ± 0.7 MV cm−1), Al0.26Ga0.74N (2.3 ± 0.6 MV cm−1), Al0.34Ga0.66N (3.1 ± 0.6 MV cm−1), Al0.41Ga0.59N (4.0 ± 0.7 MV cm−1) and Al0.47Ga0.53N (5.0 ± 0.8 MV cm−1) heterostructures were determined. The results of the field strength and field direction of all samples are in excellent agreement with values predicted by theory and a capacitance–voltage based Poisson-carrier transport simulation approach giving experimental evidence for a nonlinear increasing polarization field with Al-concentration.

Highlights

AlGaN/GaN heterostructures are commonly used for electronic and optoelectronic devices such as field-effect transistors for high power, high temperature and high voltage operation1–4) and in LEDs and laser diodes5–11) either as quantum well or as electron blocking layer
The lowsymmetry of wurtzite crystals leads to a non-zero spontaneous polarization field in equilibrium causing polarization sheet charges at AlGaN/GaN interfaces due to the large difference in spontaneous polarization.2,12) high macroscopic piezoelectric polarization fields are generated in pseudomorphically grown heterostructures due to lattice mismatch and large piezoelectric coefficients in AlGaN.2,13–15) The resulting polarization field is in the order of MV cm−116,17) and leads to a strong band bending.18–20) In order to correctly describe and understand the carrier transport and recombination in GaN/AlGaN/GaN semiconductor devices, an accurate determination of the magnitude and direction of the polarization fields is of great importance
In this paper we demonstrate the precise determination of polarization fields in AlxGa1-xN double heterostructures within a GaN matrix based on capacitance– voltage-measurements (CVM), by evaluating the changes in the depletion region width of pin diodes

Summary

Introduction

AlGaN/GaN heterostructures are commonly used for electronic and optoelectronic devices such as field-effect transistors for high power, high temperature and high voltage operation1–4) and in LEDs and laser diodes5–11) either as quantum well or as electron blocking layer. In this paper we precisely determine the polarization fields in GaN/AlGaN/GaN heterostructures based on capacitance– voltage-measurements (CVM). By evaluating the changes in the depletion region width of pin diodes caused by the polarization sheet charges at the interfaces, we are able to determine the polarization fields with high accuracy. The polarization fields were extracted from the CVM with the help of self-consistent Poisson and drift-diffusion calculations using the internal polarization charges as a fitting parameter. Both methods are based on the change of the depletion region with changing polarization field. For an accurate determination of the capacitance and in order to minimize the error, transmission electron microscopy (TEM) measurements were performed and a more accurate equivalent circuit directly related to the sample structure has been developed, tested, and applied

Experiment

Accuracy of the polarization field evaluation

Results and discussion

Conclusions